The present invention relates to a method of reducing NO.sub.x -content in flue gas during heating of coking oven. More particularly, it relates to a method of reducing NO.sub.x -content in flue gas during heating of coking ovens which has heating trains operating in pairs, high and low lying combustion stages, and a flue gas return at the height of the heating train sole (circulating stream). The invention also deals with a coking oven for performing the method.
It is known that nitric oxides formed in the coking ovens are first of all so-called thermal NO.sub.x products, whose formation rates depend approximately linearly from the product of the oxygen and nitrogen concentration in the flame, and exponentially on the flame temperature.
In the known method for reducing the NO.sub.x formation, a reduction of flame temperature during flue gas return or a reduction of the oxygen and nitrogen concentration by partial combustion were proposed.
The principle of the flue gas return is implemented in coking ovens and especially in form of the Koppers circulation stream oven. In this method through one or two openings in each second frame wall at the height of the heating sole, flue gas is mixed with the air and heating gas stream and leads first of all to reduction of the maximum flame temperature and also the reduction of O.sub.2 - and N.sub.2 -concentrations. This method leads to a considerable reduction of the NO.sub.x production rate.
The NO.sub.x reduction principle of the partial combustion is used in coking ovens in the form of a stage heating.
For the purpose of further lowering the NO.sub.x emission in coking ovens, theoretical and experimental researches have been conducted. An important result of these studies is the recognition that a combination of the NO.sub.x reduction principle, flue gas return (circulating stream heating) and partial combustion (stage heating) with two stages can lead to considerable reduction of the NO.sub.x production.
Basically the combination of the stage heating and circulating stream heating in coking ovens is known. The known researches however showed that a arbitrary combination of circulating stream heating and stage heating does not lead positively to a significant NO.sub.x reduction. Only with an optimal combination of the stage heating, circulating stream heating and arrangement of two combustion stages, a maximal NO.sub.x reduction can be obtained.
The findings obtained from the above mentioned research are summarized in the German document De-OS 3,443,976 (corresponding to U.S. Pat. 4,704,195). This document deals with rich gas ovens with two combustion stages, and with compound ovens with air and gas staging and flue gas return. Specifically, DE-OS 3,443,976 discloses supplying both the air and the gas to the heating train at a plurality of stages, i.e., at the height of the heating train sole, which constitutes a low level or first combustion stage, and at a height arranged between 35% and 65% of the height of the heating train, which constitutes a high level or second combustion stage.